I've seen worse. Here it is in 3 lines :) http://www.jimloy.com/puzz/9dots1.gif

There's a variant of this riddle where the solution is using a big brush and painting over the dots in one stroke.

I think all of these solutions are interesting and creative.

Yeah, that was my solution. Only when told they were points, not dots did I find the four line one.

You can also do it in one, if you imagine the line to lie on the surface of the earth and to wrap around the entire earth three times.

The solution doesn't violate or bend the guidelines given (using four straight lines). Assuming that some strict rules exist seems to be a cultural behavior - probably ingrained from school, where doing things creatively can get you in trouble. </sociologist>

On the other hand, if you have seen these kind of problem before, you already have a hint that the solution involves something unusual.

> The solution doesn't violate or bend the guidelines given (using four straight lines).

Yeah; which is partially my frustration. The other part is that I actually went pretty in depth compared to how much time I would expect most people to go at it. (About an hour with a pencil and paper journal. Figuring out what couldn't be the solution with "You're starting from one of the dots" as an implicit axiom.) I was actually about to write a script to brute force the solution after I ran out of ideas, until I decided to save myself the time and peek at the solution.

I think you can imagine how that turned out.

Actually it's quite easy to bruteforce by hand if you mention that point set has multiple symmetries. For example, you only need to start drawing lines from (0,0), (0,1) and (1,1). Any other combinations are derived by rotation and mirroring. After several minutes you can prove that solution in impossible under implicit presumption that every line must terminate at a dot. And after that puzzle is really easy.